Concentrated (e.g. above 72% by weight) dispersions of calcium carbonate ground to a particle size such that at least 87% by weight of the particles are less than 2 .mu.m in size have a tendency to gel on standing. In the invention this tendency is minimized or eliminated by using, as dispersing agent, a copolymer of from 5 to 75% by weight AMPS and 95 to 25% by weight acrylic acid, or a water soluble salt thereof, having a molecular weight in the range 1000 to 20,000.